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Heat pump indoor coil freezing up.

I have a 3 ton Trane Weathertron 1200 XL that froze the indoor coil to where it completly blocked all airflow. When I discovered it I turned the system off but kept the blower running until it thawed out. I opened the air handler and removed the filters to make sure it was completely thawed then turned the system back on and the heating resumed as normal. I called my AC Tech and he said it sounds like a defrost control problem where the system would initiate a defrost cycle but not terminate it. He came out and tested the defrost control. It is a demand type system that uses a temperature differential between two sensors. The control board checked ok by doing the "Forced Defrost" and the blinking light on the board indicated normal. The sensors were removed and resistance checked ok at three different temps. The refrigerant pressures checked good and the filters are clean. He left saying everything is normal. That night it froze again! The next day he again checked the defrost control and cycled the reversing valve several times but could not duplicate the problem. Again it froze up.
I live in the mountains in TN and we tested it during the day with 40-45 temps. It gets in the high 20's at night when the coil freezes.
Primary heating is with wood using a fireplace insert. The heat pump is a backup but I leave the indoor blower running continuous to help circulate the heat. Thanks in advance for any help.

The defrost system is to defrost the OUTDOOR coil, not the indoor coil. The indoor coil should be warm in the winter, not cold. If it is getting cold and freezing, you are in cooling mode which you shouldn't be in cold weather.

It's definitely sticking/hanging up in the defrost mode. Since it's so intermittent and nobody can duplicate the problem, it's time to R&R. I'd start with the defrost board and sensors. If that doesn't do it, you're like in for a reversing valve. Not a good project in cold weather. Electric heaters? You might want to use them if it's not the board or sensors.

Forgive my confusion, but how can the Indoor Coil freeze up with the Heat Pump in the Heat mode? The Indoor Coil is where the High Pressure/Temp Refrigerant Gas goes to be cooled down to become a High Pressure Liquid. Thus giving off its heat to warm the Indoor air.

The only senerio I can see that the Indoor Coil Freezing up is during a Defrost Cycle that terminates, but the Reversing Valve does not cycle back into the Heat Mode. That would cause the system to be in the Cooling Mode. Where the Indoor Coil would be absorbing heat, but due to the low temp Inside/Outside. It would drop the coil below freezing and Freeze Up. So I would expect a sticking Reversing Valve to be the issue.

main source of heat is wood and stat is auto, room gets too hot and the A/C comes on to cool to set temp.

just a thought

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from an excerpt by Paul Jacob in Sun City, AZ